Stacked controls should expose a method/attribute to allow for dynamic ordering

[davidrleonard]

See #29 for the precursor to this request.

Right now, controls are stacked in the same position in reverse DOM order automatically. If it seems useful for developers, we should expose an explicit order attribute or some other way to allow developers to dynamically re-stack controls on the map without detaching and re-attaching them.

[stale]

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